There are abundant fossil fuels which contain low melting point alkaline compositions (usually alkali metal salts), particularly lignite and salty brown coals. However only a small portion of such fossil fuels are utilized commercially because of the difficulties in producing energy from burning the lignite or the like. Typically, lignite is burned on a grate in a boiler furnace. However this requires high combustion temperatures, in the range of 1200.degree.-1500.degree. C., which causes sintering of the fuel. At these temperatures, the sulfates and chlorides evaporate, causing condensate on the walls of the furnace and other surfaces, and enhancing the formation of deposits on the boiler heat transfer tubes causing corrosion of the tubes and poor heat transfer. In order to deal with this problem, typically the furnaces must be frequently shut down and the deposits removed from the heat transfer tubes, something that is difficult to do.
While fluidized bed reactors are known to have many advantages over conventional boiler furnaces, in the past it has not been considered practical to burn many types of lignite and salty brown coal in fluidized bed reactors. This is because the alkalines in the lignite cause agglomeration of the bed material. The higher the alkali metal salt content of the fuel, the lower the agglomeration temperature is.
According to the present invention a method is provided which solves the long felt need of being able to effectively burn fuel containing alkaline compositions, such as lignite, and recover energy therefrom in a relatively simple and straight-forward manner. According to the present invention, this is accomplished utilizing a circulating fluidized bed reactor. Fluidized bed reactors are typically operated in the temperature range of about 750.degree.-950.degree. C. At the lower end of the temperature range the combustion of the fuel deteriorates, and at the upper end of the range the risk that the bed material will sinter increases.
According to the invention it is possible to burn solid fuel having low melting point alkaline compositions in a fluidized bed reactor by: using a circulating fluidized bed reactor, which has a relatively uniform temperature throughout the reaction chamber; and by the addition to the reaction chamber of a reactant material capable of reacting with the low melting point alkaline compositions of the fuel to produce high melting point alkali metal compounds during combustion. The alkali metal compounds produced during combustion have a high enough melting point so that the reactor may be operated at a desirable temperature (within the range of about 750.degree.-950.degree. C.) without melting. In this way agglomeration of the bed material, sintering of the fuel, and enhanced formation of deposits on operable components of the reactor are prevented.
The reactant material utilized according to the invention comprises an oxide, or a hydroxide which is converted to an oxide during combustion, of the group consisting of aluminum, calcium, magnesium, silica, iron, titanium, and mixtures of two or more of aluminum, calcium, magnesium, silica, iron, and titanium. If silica oxide is used, it desirably is used with a metal oxide. Typically sufficient metal oxide is added so that the ratio of metal of the metal oxide to metal of the alkali metal salts in the fuel is at least about 1.0. Preferably, the reactant material comprises kaolin (clay) which includes oxides of all of silica, aluminum, iron, titanium, calcium, and magnesium, and which reacts with the fuel and the circulating bed material to form high melting temperature sodium compounds. Typically the molar ratio of aluminum in the kaolin to sodium plus potassium in the fuel is at least 1.0.
It is desirable to also add limestone with the reactant material in order to absorb sulfur. Further, since the combustion of the fuel is for the purpose of producing useful heat energy (which may be transformed into steam energy, electricity, or the like), it is desirable to recover heat energy directly from the reaction chamber utilizing heat recovery apparatus disposed on the surface of, or in, the reactor chamber. Such heat recovery apparatus, which is conventional per se, has had minimal utility in the past when lignite was the fuel due to the build up deposits on the heat recovery surfaces. However in view of the fact that the formation of deposits on the heat recovery tubes is minimized according to the invention, such apparatus can be effectively utilized in the reaction chamber itself.
After start-up, a circulating fluidized bed operated according to the invention has little sand or other bed-forming constituents in the fluidized bed. While some sand, or other accessory bed material is added during start-up, once a steady state condition is achieved the circulating bed material comprises mainly the lignite fuel, kaolin, and ash; still agglomeration of the bed material does not occur. The reactor is operated, including by withdrawing and recovering heat directly from the reaction chamber, so that the temperature in all parts of the reaction chamber is between about 750.degree.-950.degree. C., and specifically is lower than the melting temperature of the alkali metal compounds formed by the reaction of the kaolin (or the like) with the alkali metal salts in the fuel, during combustion.
It is the primary object of the present invention to provide a simple yet effective method for burning fuel containing low melting point alkaline compositions, to produce and recover heat energy. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.